1. Field of the Invention
The present invention relates to an electronic device having a sound output module which produces a smaller pop-up sound upon startup.
2. Description of the Related Art
An electronic device having a sound output module or circuit in it is configured such that upon startup a bias of an output amplification circuit or a like circuit is gradually increased at a certain time constant in order to prevent occurrence of (or reduce) a pop sound (a so-called “snap sound”) which is offensive to the ears.
FIG. 12 is a view showing an example configuration of an electronic device having a related-art sound output module. In the drawing, a sound signal processing IC 200 has a preamplifier 10 and output amplifiers 20, 20A and subjects, to signal processing, an input signal Vin incoming from a signal input terminal Pin, thereby outputting an output signal Vout from a signal output terminal Pout or PoutA. The signal Vout output from the signal output terminal Pout is supplied to a speaker Sp by way of a coupling capacitor Cc. As for the signal output from the signal output terminal PoutA, it is used for a sound output device, such as an earphone.
The preamplifier 10 has an operational amplifier 11, a resistor 13 being coupled between the input terminal and the output terminal thereof, and an input resistor 12. A first bias voltage Vb1 is supplied to the other input terminal of the operational amplifier 11. This preamplifier 10 performs the function of impedance matching or buffering. The output amplifier 20 has an operational amplifier 21, a resistor 23 connected across input and output terminals thereof, an input resistor 22, and an input signal switch 24. A bias voltage Vb1 is supplied to the other input terminal of the operational amplifier 21. The output amplifier 20A is analogous to the output amplifier 20.
A first bias circuit 30 supplies a bias voltage Vb1 which is generated at a node between resistors 31, 32 serially connected between a source voltage Vdd and the ground, to the preamplifier 10, the output amplifiers 20, 20A, and other required elements. An external capacitor 33 is connected in parallel with the resistor 32 via a capacitor connection terminal Pc1.
When the source voltage Vdd is applied to the sound signal processing IC 200 shown in FIG. 12, the bias voltage Vb1 gradually increases from a zero voltage and reaches a predetermined voltage with lapse of time. Therefore, although the preamplifier 10 and the output amplifier 20 are in an operable state, fluctuation in a d.c. voltage on an output side of the coupling capacitor Cc can be suppressed to a low level, thereby diminishing a pop sound. Thus, a bias voltage can be supplied from a bias source that is commonly used for a plurality of amplifiers requiring a bias voltage so that the entire configuration of the sound output module can be simplifed.
A sound output module configured to supply a bias voltage from a common bias source to a plurality of circuit blocks has been widely used as disclosed in Japanese unexamined patent publications JP Hei 8-18342 or JP Hei 5-136634.
A portable cellular phone is taken as an example of the electronic device including such a sound apparatus. As shown in FIG. 12, the portable cellular phone has a plurality of output terminals, such as the speaker signal output terminal Pout for outputting an output signal from the output amplifier 20; the earphone signal output terminal PoutA for outputting an output signal from the output amplifier 20A; and an external output signal output terminal PoutB for outputting an output signal from the preamplifier 10. In general, however, a portable cellular phone uses a battery as its power source. Therefor, it is required to minimize the power consumption so as to extend the operation time of the battery power source as long as possible. For overcoming this requirement, in a related art, the portable cellular phone is configured such that an ON-OFF control signal Vpon, which is input from ON-OFF signal input terminals Pon and PonA, is provided with each output amplification circuit so as to activate only an output amplification circuit to be used at that point in time and deactivate an output amplification circuit which is not used. As the result of this configuration, upon being activated during the course of using the portable cellular phone, the output amplification circuit emits a pop sound.